Metal protection

ABSTRACT

Metals for use in glass-making furnaces and which are susceptible to oxidation at furnace operating temperatures, especially iridium or molybdenum, are protected by applying at least (200) microns thickness of a coating formed of metal oxide particles in a metal oxide matrix. Oxidation, measured by weight loss, is significantly reduced.

The present invention concerns improved metal protection, and moreespecially concerns the protection of metals from oxidation at elevatedtemperatures.

The quality of glass products is markedly affected by thecharacteristics of the vessels in which the glass is originally melted.High volume, relatively low quality glass is easily obtained by meltingin ceramic refractory vessels and furnaces, with the best of theseproviding excellent resistance to the corrosive nature of the moltenglass, and hence good longevity. This is important since the failure ofthe refractories is usually the instigator of expensive furnacerebuilds, and anything which can inhibit this corrosion is potentiallyvaluable. Platinum claddings and other forms of platinum coatings havebeen introduced to protect the most critical parts of the furnace, toimprove longevity, and glass quality. In the ultimate scenario the wholefurnace is protected with platinum and the very highest quality glass,with very low inclusion count, is produced.

The drive for excellence in respect of glass quality is nowhere greaterthan for glasses requiring optical transparency characteristics, such asLCD and flat glass. Inevitably furnaces constructed using platinum orplatinum alloys are intrinsically very expensive. Moreover, theprocessing temperatures in the melting section of such furnaces are veryclose to the melting temperature of the metal containment. Althoughalloying platinum with rhodium and other platinum group metals can givesome considerable increase in useable temperature range, this isinsufficient to be practicable. It is very desirable therefore to use analternative metal or alloy, with a higher melting point, higherstrength, better creep resistance and excellent resistance to the moltenglass corrosion. Molybdenum, rhenium, tantalum, niobium and tungsten allhave these basic characteristics, but each has the fatal limitation thatit oxidises rapidly at elevated temperature, in some cases as low as450° C. Indeed as the temperature is raised above this, the rate ofoxidation increases until by 1000° C. the metals can be considered to be“burning”.

Iridium has similar physical and mechanical characteristics but is muchless susceptible to oxidation attack than the aforementioned metals,such that oxidation does not become significant until at least 1200° C.Indeed for short periods of time, iridium car used unprotected in air attemperatures as high as 2200° C., although for durability over severalyears the oxidation rate would still represent a concern.

Thus the problem to be solved is how to decrease the oxidation rate ofiridium, or indeed other high melting temperature metals, and theiralloys, to achieve a desirable length of life at very high temperaturewhen exposed to oxygen-containing atmospheres. It is known to use inertatmospheres to protect iridium vessels operating at high temperatures.

Certain metals, including molybdenum, have been coated with ceramic byflame or plasma spraying, but such coatings have limited protectivenessin practice. Indeed, such coatings tend to be inherently porous and donot resist ingress of air/oxygen well. Molybdenum which has been flameor plasma sprayed with alumina and/or zirconia has to be protected fromair with platinum fabrications with any free space meticulously expungedof air. It can be protected with silicide layers produced by hydrogenreduction of silane, but these are suited only to the short timesrequired for set up of electrodes prior to immersion in glass.

The present invention offers, at least in its preferred embodiments, amethod for restricting the rate of oxidation of iridium and its alloysand to a lesser degree similar metals and alloys, at temperatures of atleast 1600° C. and in some cases as high as 1800° C.

Accordingly, the present invention provides a protected metal, having asubstrate selected from the group consisting of platinum group metals,molybdenum, rhenium, niobium, tungsten and alloys of any of such metals,comprising, on a surface exposed to an oxygen-containing atmosphere, acontinuous composite coating of at least 200 microns thickness,especially of 500 to 1000 microns, which coating comprises metal oxideparticles in a metal oxide matrix.

The invention also provides a method of protecting a metal substrateselected from the group consisting of platinum group metals, molybdenum,rhenium, niobium, tungsten and alloys of any of such metals surface fromoxidation in an oxygen-containing atmosphere at elevated temperature,comprising applying to said metal substrate a coating of metal oxideparticles together with a metal oxide matrix or a metal matrixprecursor.

The metal oxide particles are suitably of varying sizes, suitablediameters being diameters in the range of 50 to 100 microns, but this isnot believed to be critical (although size can have an effect on ease ofcoating application). As well as conventional particulate ofapproximately spherical shape, the invention includes the use of ceramicfibres as an alternative to conventional particulate or in admixturewith conventional particulate. The metal oxide may be one of or,preferably, a mixture of, alumina, silica and zirconia. Such particlesmay be mixtures of individual oxides or may be mixed oxides. Theparticles may contain one or more other metal oxides including one ormore of iron oxide, one or more rare earth metal oxides, magnesia,titania and hafnia. Other components may also be present, providingthese are beneficial or do not significantly adversely affect thestability of the particles under the operating conditions. Preferredparticle compositions in our initial trials contain from 45 to 90% by wtAl₂O₃, 10 to 45% by wt of SiO₂, and less than 1% by wt of Fe₂O₃ andother metal oxides, from a total composition of 100%.

The metal oxide matrix should be physically compatible with the metaloxide particles, such that the coating has no physical cause fordegradation during use. Desirably, the matrix is such that during itsformation, there is a chemical bond between the particles and thematrix. The preferred matrix precursor is a silicate-based solution suchas an aqueous solution of sodium silicate. Other precursors, or othercomponents, are also to be considered.

A ceramic paste or slurry containing particles and matrix-formingmaterial may be prepared to allow easy application to the metal surfaceto be protected using a spatula or trowel. In refined versions of theinvention, some compositions may be applied by brushing, spraying,dipping or even a combination of these. Additionally some forms of thecoating may be applied in two parts, by which an aqueous component ofthe matrix-forming precursor is deposited by spraying and brushing andthe oxide components by sprinkling or stucco treating. In this latterembodiment, multiple layering is generally required to build the correctminimum coating thickness. This process is very reminiscent of theprocess technique used from preparing shell moulds for metals casting,e.g. jewellery and gas turbine blades.

After deposition of the coating on the metal, it is desirable to dry thecoating in a controlled manner to reduce or eliminate any formation ofgas bubbles. Initial tests indicate that firing of the coated metal isnot essential, but may be desirable to develop the full hardness andstrength of the coating. Such firing should be carried out at atemperature of at least 700° C.

It is, of course, desirable to ensure that the coating is continuous andnon-porous. In certain circumstances at least, the application of two,three or more coatings is desirable. In certain circumstances, if thereis damage caused to the protective layer, for example during thermalcycling, it may be possible to apply a repair layer of oxide.

Suitable slurries for use in the invention are available commercially,and they have been marketed for joining, protecting and repairingceramics by companies such as Sheffield Refractories (Jonsett H A) andFortafix Ltd. (Fortafix). Such slurries have not previously been used,to the best of our knowledge, for such an application of protectingmetals of the type that this invention is concerned with, and theirparticular suitability to protection of iridium, molybdenum and similarmetals at high temperature had not previously been recognised.

The invention will now be described by reference to the followingExamples.

EXAMPLE 1

A variety of ceramic slurries were prepared or purchased and applied toiridium coupons which had previously been grit blasted and cleanedsonically. Each slurry was applied by painting in two phases, with eachphase being allowed time to dry under ambient conditions before beingturned and the remainder painted. Two complete layers were applied toeach sample coupon, but the final thickness was determined by the natureof each mixture such as viscosity.

The coated coupons, together with an unprotected control coupon ofiridium, were placed flat on upturned high purity alumina tube to allowfor maximum airflow around the samples, inside a muffle furnace. Thesample coupons were then heated at 1200° C. in air for 336 hours, andallowed to cool.

After the heating cycle, the ceramic coatings were removed mechanically,and the coupons were lightly peened to remove any residual ceramic andoxide. The weight losses for each coupon, measured in mg/sq mm, wereestablished, and are shown in FIG. 1. The unprotected coupon lost themost weight, and all the coatings provided some protection. The coupons6 and 7 showed the least protection; the slurries for these werewater-based, and all other slurries contained sodium silicate.

EXAMPLE 2

As part of the experimentation, small coupons of iridium were cut andcleaned. After weighing, each coupon was placed on a bed of ceramicpowder within an alumina crucible. The crucible was then filled with thesame ceramic powder, so that the coupons were completely immersed. Inone case, the best slurry from Example 1 was used to cover the coupon.The crucibles and their contents were placed in a small muffle furnaceand the temperature was raised to 1600° C., which was maintained for 168hours.

The crucibles and contents were removed, and each coupon was carefullyrecovered. Any residue of ceramic was cleaned by gentle peening, and thecoupons wrre reweighed. The weight differences, as a function of surfacearea, are plotted in FIG. 2. The most effective barrier to weight losswas that of “Slurry 3” from Example 1.

EXAMPLE 3

Three iridium coupons were cut from the same rolled iridium sheet. Eachwas coated with “Slurry 3” and allowed to dry, two of the coupons weregiven a second coat, and one of those had a third coat applied. Thecoupons were then heated in air at 1200° C. for 336 hours, as inExample 1. The results are shown in FIG. 3, and it is clear thatmultiple coatings improve the protection significantly. Indeed for threecoatings, the improvement is an order of magnitude better than thesingle coating. Thermal cycling of coated metals can demonstrate somespalling of thicker ox but microscopic examination of the surface showsa residual layer of oxide is retained, which continues to give a measureof protection.

EXAMPLE 4

Initial tests were carried out by coating coupons of tantalum and ofmolybdenum using a single coating of “Slurry 3”, and exposing the coatedcoupons to air at 1400° C. for 48 hours. The tantalum coupon exhibitedcatastrophic oxidation, with the increase of volume causing spalling ofthe oxide layer. However, the molybdenum coupon appeared to besignificantly better protected, with some small areas probablycoinciding with flaws in the coating. Optimisation of coatingcharacteristics and the underlying metal needs to be carried out. Thistest indicated, however, that useful short-term protection of molybdenumis certainly possible, for example for installation purposes, or theinvention may be used together with a platinum cladding.

1. A protected metal, having a metal substrate selected from the groupconsisting of platinum group metals, molybdenum, rhenium, niobium,tungsten and alloys of any of such metals, comprising, on a surfaceexposed to an oxygen-containing atmosphere, a continuous compositecoating of at least 200 microns thickness, which coating comprises metaloxide particles in a metal oxide matrix.
 2. A protected metal accordingto claim 1, wherein the metal substrate is iridium or an alloy thereof.3. A protected metal according to claim 1, wherein the metal substrateis platinum or an alloy thereof.
 4. A protected metal according to claim1, wherein the metal substrate is molybdenum, rhenium, niobium, tungstenor an alloy thereof.
 5. A protected metal according to claim 1, whereinthe metal oxide matrix is derived from a silicate salt.
 6. A protectedmetal according to claim 1, wherein the metal oxide particles comprisefrom 45 to 90% by wt Al₂O₃, 10 to 45% by wt of SiO₂, and less than 1% bywt of Fe₂O₃ and other metal oxides, from a total composition of 100%. 7.A protected metal according to claim 1, wherein the coating has beenformed from two or more layers of coating.
 8. A method of protecting ametal substrate selected from the group consisting of platinum groupmetals, molybdenum, rhenium, niobium, tungsten and alloys of any of suchmetals surface from oxidation in an oxygen-containing atmosphere atelevated temperature, comprising applying to said metal substrate acoating of metal oxide particles together with a metal oxide matrix or ametal oxide matrix precursor.
 9. A method according to claim 8,comprising the application of two or three layers of the coating.
 10. Amethod of making glass comprising using a protected metal according toclaim
 1. 11. A protected metal according to claim 1, wherein thecontinuous composite coating is between 500 and 1000 microns inthickness.